Electrophotographic copying material and its manufacturing method

ABSTRACT

AN ELECTROPHOTOGRAPHIC COPYING MATERIAL COMPRISING A SUPPORT AND A LIGHT-SENSITIVE OR PHOTOCONDUCTIVE LAYER FORMED ON ONE SURFACE OF SAID SUPPORT, SAID PHOTOCONDUCTIVE LAYER BEING FORMED IN SUCH A WAY THAT A PHOTOCONDUCTIVE LAYER-FORMING SOLUTION-WHICH IS PREPARED BY DISPERSING A PHOTOCONDUCTIVE SUBSTANCE IN A BASIC AQUEOUS SOLUTION CONTAINING A POLYBASIC ACID MONOESTER OF A POLYVINYL ACETATE AND ALSO A VOLATILE BASIC SUBSTANCE-IS APPLIED ONTO SAID ONE SURFACE OF SAID SUPPORT AND THEN DRIED.

un 5, 1971 sAKAssl-umzu ELECTROPHOTOGRAPHIC COPYING MATERIAL AND ITS MANUFACTURING METHOD Filed June 6, 1968 LIGHT ON CHARGE OFF CHARGE ON wmwmmo 6w4wwm All E iJsEEom wufimnm TlME(SEc) United States Patent Oifice 3,585fl27 Patented June 15, 1971 U.S. Cl. 961.5 4 Claims ABSTRACT OF THE DISCLOSURE An electrophotographic copying material comprising a support and a light-sensitive or photoconductive layer formed on one surface of said support, said photoconductive layer being formed in such a way that a photoconductive layer-forming solutionwhich is prepared by dispersing a photoconductive substance in a basic aqueous solution containing a polybasic acid monoester of a polyvinyl acetate and also a volatile basic substanceis applied onto said one surface of said support and then dried.

BACKGROUND OF THE INVENTION (a) Field of the invention The present invention is concerned with an electrophotographic copying material provided, on one surface of its support, with a photoconductive layer wherein a novel water-soluble polybasic acid monoester of a polyvinyl acetate is used as the binder resin for the components constituting said photoconductive layer, said electrophotographic copying material being featured by its excellent electrostatic characteristics which are exhibited even in an atmosphere of high temperature and high relative humidity.

(b) Description of the prior art In the electrophotographic copying materials of the prior art, it has been the practice to use zinc oxide as the photoconductive substance which is one of the components constituting the light-sensitive layer (photoconductive layer) of such a copying material, and also to use in general, as the binder resin for binding the photoconductive substance and other components to the support, a resin-for example, silicone resin, alkyd resin, acrylic resin, butadiene copolymer resin or phenol resinwhich is soluble in an organic solvent such as toluol or xylol. In view of the fact, however, that the light-sensitive layers were formed in the past in such a way that a photoconductive substance and a binder resin were dispersed and dissolved in an organic solvent such as the one described above, this light-sensitive layer-forming operation naturally involved the possibility of hazards such that the operator was susceptible to inhaling the poisonous vapor of the organic solvent and that the volatile solvent would easily catch fire and explode. For the prevention of the foregoing hazards from taking place, the copying material manufacturers have been spending an enormous sum of money in the installation or improvement of the equipment intended to avoid the occurrence of such hazards.

From the consideration that the aforesaid hazards can be altogether disregarded if the binder consists of a resin which is soluble in water, there have been proposed a number of water-soluble resins for use as the binder to be contained in the light-sensitive layers. However, the electrophotographic copying materials which were prepared according to these proposals had the drawbacks in their electrostatic characteristics such that the light-sensitive layers which were impressed with an electric charge exhibited a low charge potential and also that they showed a large amount of dark decay, which is the decrease in the potential exhibited by the copying materials during the period in which they are left to stand after having been impressed with an electric charge. As such, the electrophotographic copying materials of the prior art were too unsatisfactory to be used for practical purposes.

SUMMARY OF THE INVENTION It is, therefore, a primary object of the present invention to provide an. electrophotographic copying material having a light-sensitive layer wherein a binder which is soluble in alkaline aqueous solutions is used as one of the components for constituting the light-sensitive layer.

Another object of the present invention is to provide an electrophotographic copying material which exhibits electrostatic characteristics which are substantially the same in degree and extent as those exhibited by the conventional electrophotographic copying materials wherein a binder which is soluble in organic solvents is used as a component of the light-sensitive layers.

Still another object of the present invention is to provide an electrophotographic copying material which exhibits superior electrostatic characteristics even in an atmosphere of high temperature and high relative humidity.

The present invention has been discovered based on the knowledge which was obtained from extensive research conducted on the electrostatic characteristics of the electrophotographic copying materials which were produced by forming-on one side of a support consist ing of a metal plate or a paper sheet whose reverse side having been processed to have an electroconductivity a light-sensitive layer with a combination of a zinc oxide which is a photoconductive substance and a water-soluble resin as the binder, and also as the result of a number of experiments in which a variety of water-soluble resins were used. More specifically, it has been discovered that in case particularly a polybasic acid monoester of a polyvinyl acetate, of all other water-soluble resins, is used as the binder resin, the resulting electrophotographic copying material exhibits excellent electrostatic characteristics which are not inferior to those exhibited by the conventional electrophotographic copying materials utilizing the binders which are soluble in organic solvents.

The polybasic acid monoesters of polyvinyl acetate which are used in the present invention refer to those substances which are obtained from a reaction between a polybasic acid which is represented by such acids as maleic acid and itaconic acid and a polyvinyl acetate. It is to be noted, however, that the substances which are obtained from the foregoing reaction are featured by the fact that the unsaturated radicals of the polybasic acid used are retained as they were in these substances, and accordingly, the substances which are obtained from said reaction differ in chemical properties from the so-called ordinary copolymers of polyvinyl acetate and polybasic acids. For example, the monoester which is obtained from the reaction between maleic acid and a polyvinyl acetate will have the following chemical structure:

O=OCH= CH The polybasic acids which are referred to in the present invention are directed to those unsaturated carboxylic acids having two or more carboxyl radicals in one molecule. These unsaturated carboxylic acids include, for example,

fumaric acid, in addition to the aforesaid maleic acid and itaconic acid.

Various kinds of polybasic acid monoesters manufactured by the Nippon Synthetic Chemical Industry Co. Ltd. of Japan are commercially available also.

According to the present invention, the use of those monoesters which are produced as the result of the coupling of a polyvinyl acetate to one of the aforesaid polybasic acids of an amount ranging from 2% to 30% by mole are most desirable. As for the photoconductive substances for being dispersed in resins consisting of such polybasic acids as those described above to prepare lightsensitive layer-forming solutions, there can be employed all of those known photoconductive substances such as zinc oxide, titanium oxide, zinc sulfide, zinc selenide and cadmium sulfide. It is to be noted also that, according to the present invention, the inclusion-in the light-sensitive layers of the electrophotographic copying materials of those dyestuffs such as Rose Bengal, Fluorescein, Bromophenol Blue, Tetrabromophenyl Blue, Methylene Blue and Methyl Violet, which are known as the sensitizers for use in electrophotography in general not only will not be harmful, but also are even desirable. Also, as the supports which can be applied to the present invention, the use of paper sheets, films or the like which have been given an electroconducting treatment by the application of or by being impregnated with an electroconductive substance such as surface active agents, in addition to the use of metal plates, is also desirable.

The binder resins which consist of those aforesaid polybasic acid monoesters of polyvinyl acetates and which can be applied to the present invention all have a property such that they become water-soluble for the first time by the addition thereto of a basic substance such as alkali or amines. It is important to note that, in case the aforesaid binder resins are made to be soluble in water by the addition thereto of at least one of the volatile basic substances, such as ammonia, triethylamine, trimethylamine, diethylamine, dimethylamine, ethylamine methylamine, methylethylamine, diisopropylamine and methyldiethylamine, of all the substances which are called basic substances, these volatile substancessuch as ammonia or trimethylamine which are made to be contained in the light-sensitive layers of the electrophotographic copying materials which are manufactured by the use of the aforesaid mixed solution will evaporate and disappear during the drying step in the process of forming the light-sensi tive layers, and accordingly, the binder resins will change to non-soluble in water and at the same time will gain a markedly increased electric resistance. For this reason, it is all the more desirable in the present invention to make the binder resins soluble in water by the use of these basic substances which are volatile. While there is no restriction in particular with respect to the temperature which is employed in the aforesaid drying step, it is desirable to carry out the drying at a temperature ranging from 100 C. to 200 C. By performing the drying of the applied solution at a temperature within the foregoing range, there are obtained light-sensitive layers having excellent electrostatic characteristics. Furthermore, in case the binder resins are made to be soluble in Water by the addition thereto of a water-soluble substance such as hydrogen peroxide which has a catalytic action, together with one of the aforesaid desirable basic substances, the resulting light-sensitive layers of the electrophotographic copying materials which are manufactured by the use of these substances will exhibit better electrostatic character istics. This effect is considered to result from the fact that, owing to the reactivity of the unsaturated radicals contained in the polybasic acid which is united to the polyvinyl acetate, there occurs a reaction between the monoester molecules causing them to be united to each other, and that, as a result, these monoester molecules are converted, during the drying step in the process of forming light-sensitive layer, to a polymer having a three dimensional chemical structure.

As has been described, the present invention contemplates the provision of an electrophotographic copying material by first applying, onto one surface of a support, an aqueous solution of light-sensitive layer-forming components which are comprised essentially of a photoconductive substance and a binder resin consisting of a polybasic acid monoester of a polyvinyl acetate, and then drying the applied solution. The electrophotographic copying material thus obtained not only will exhibit superior electrostatic characteristics in a normal atmosphere, but also will exhibit equally superior electrostatic characteristics even in an atmosphere of high temperature and high relative humidity. In this respect, the electrophotographic copying material of the present invention possesses properties which are superior to those of the commercially available electrophotographic copying materials of the prior art.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a graph containing curves showing the electrostatic characteristics of the electrophotographic copying materials of the present invention and curves indicating that of the control electrophotographic copying materials which are intended to be used for the comparison of the abilities of the copying materials.

In the drawing, the curves 1 and 1 represent the electrostatic characteristics of the electrophotographic copying materials obtained in the Example 1 which will be described later, while the cnrves 2 and 2 and the curves 3 and 3' represent the electrostatic characteristics of the control copying materials, respectively. The conditions under which these curves were obtained will be described in detail in Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will hereunder be described in further detail in connection with the following examples.

Example 1 A photoconductive substance-dispersed solution (A) consisting of:

Zinc oxide A dispersing agent (sodium salt of polyacrylic acid) 0.3

Water 50 and an aqueous solution (B) of a binder resin which is composed of:

Gr. Maleic acid monoester of polyvinyl acetate (a 10% by mole monoesterified compound produced by the Nippon Synthetic Chemical Industry Co., Ltd. which is sold under the trade name of M-45 (X6)) 20 Hydrogen peroxide solution (35% or over) 0.5 Ammonia water (28%) 20 A sensitizer dye (Rose Bengal) 0.04 Water 50 were prepared. By dispersing and mixing the aforesaid respective solutions (A) and (B) by the use of a homogenizing mixer for 10 minutes, there was prepared a solution for forming light-sensitive layers. Onto the front sides of the support sheets made of an art paper with the reverse sides having been processed to have electroconductivity was applied the aforesaid solution for forming light-sensitive layers in such a way that the amount of the solution applied was in the range of from 30 gr./m. to 40 gr./m. and thereafter the applied solution was dried at a temperature ranging from C. to C. for a period ranging from 5 to 10 minutes in a drier. Thus,1 electrophotographic copying materials were obtaine Also, for the comparison of the electrostatic characteristics with those of the product of the present invention, control electrophotographic copying materials (a) were prepared in the following manner. First, a binder resin solution was prepared by dissolving a copolymer of a vinyl acetate and crotonic acid (a basic acid) in a mixed solution of water and an ammonia water, while on the other hand, preparing a photoconductive substance solution in which zinc oxide and a dispersing agent were dispersed in water. By mixing these two solutions together and using this mixed solution in the same way as described in connection with the production of the present invention, said control copying materials were obtained. Furthermore, still other control copying materials (b) were also prepared by mixing Latex which was a copolymer of styrene and butadiene in a photoconductive substance-dispersed solution having a composition identical to that used in the production of the copying materials of the present invention, and thereafter using the resulting solution in a manner similar to that employed in the manufacture of the product of the present invention.

Then, the electrostatic characteristics of both of the electrophotographic copying materials of the present invention and the aforesaid two types of control copying materials (a) and (b) were measured. The result of the measurements will hereunder be described by referring to the accompanying drawing. The drawing is a graph of the electrostatic characteristics in which are plotted the changes occurring in the potential of the light-sensitive layers during the period from the time the electrophotographic copying materials thus obtained were impressed with an electric charge (charge on) by corona discharge till the time immediately after they were exposed to light (light on). In the drawing, the curves 1 and 1 show the electrostatic characteristics of the copying materials of the present invention; the curves 2 and 2' show those of the control copying materials (a), and the curves 3 and 3' show those of the control copying materials (b), respectively. The curves 1, 2 and 3 represent the results of the measurements which were conducted on these copying materials at 20 C. and a R.H. of 65%, while the curves 1, 2' and 3' represent the result of the measurements which were performed on the copying materials at 30 C. and a R.H. of 80%.

As will be clearly noted from the drawing, the copying materials of the present invention show a quicker rise of the potential when the copying materials are impressed with an electric charge, and show a smaller amount of dark decay which means the attenuation of the potential from right after the suspension of the charging (charge off) till the time the copying materials are exposed to light, and also show quicker light decay which is exhibited when the copying materials are exposed to light, as compared with the control copying materials, and thus, it will be understood clearly that the electrophotographic copying materials of the present invention sufficiently satisfy the electrostatic characteristic qualifications which are required of electrophotographic copying materials. It was also noted in this example that the control copying materials (2.) exhibited satisfactory abilities in a normal atmosphere (which means a temperature of 20 C. and a R.H. of 65%) as are noted from the curve 2, but their abilities are markedly reduced, as will be noted from the curve 2, when subjected to an atmosphere of a high tem perature and a high RH. (meaning a temperature of 30 C. and a relative humidity of 80%). It was also noted that the control copying materials (b) were altogether unsuitable for being used for practical purposes.

The aforesaid respective types of electrophotographic copying materials were then used in taking copies by the use of an electrophotographic copying apparatus (which is manufactured and sold by K. K. Ricoh of Japan under the trade name of Ricoh Electronic Printer BS-l), with the result that very clear copied images were formed on the copying materials of the present invention (the reflection density of the copied images measured by a densitometer manufactured by Photovolt Company of USA. was noted t be 1.50 or over), in contrast to the slightly obscure copied images which were formed on the control copying materials (a), namely, the reflection density of the copied images was in the order of 0.8. Also, in case the copying operation was conducted in an atmosphere of a high temperature and a high relative humidity, the copying materials of the present invention showed the formation of satisfactory copied images, whereas there was formed entirely no copied image on the control coping materials (a).

Example 2 A binder resin solution which was composed of:

Gr. Fumaric acid monoester of polyvinyl acetate (a 12% by mole monoesterified compound manufactured and sold under the trade name of M-50 (Z7) the Nippon Synthetic Chemical Industry Co.,Ltd.) 20 Hydrogen peroxide solution (35% or over) 0.6 Ammonia water (28%) 10 Diethylamine 5 Ethanol 10 A sensitizer dye (Rose Bengal) 0.004 Water 50 was mixed in a photoconductive substance-dispersed solution having a composition identical to the solution (A) used in Example 1, and the resulting mixed solution was dispersed thoroughly by the use of a homogenizing mixer for 15 minutes. Thus, a solution for forming light-sensitive layers was prepared. This solution was applied onto the front sides of the support sheets, which had been given a surface treatment with polyvinyl alcohol at the surfaces, in such a way that the amount of the solution applied was 35 gr./m. Then, the applied solution was dried at 13 C. for 10 minutes in a drier, and as a result, electrophotographic copying materials were prepared. The electrophotographic copying materials thus obtained were regulated as regards their moisture content in an atmosphere having a temperature of 20 C. and a relative humidity of 65% for 12 hours. Thereafter, the resulting copying materials were measured as regards their electrostatic characteristics in a manner similar to that described in connection with Example 1 and furthermore, they were subjected to a copying operation in the same way. It was noted that these copying materials had electrostatic characteristics which were of the same order as those exhibited by the copying materials obtained in Example 1, and furthermore, the copying materials obtained in this instant example gave very clear copied images.

For comparison, control electrophotographic copying materials were prepared by the use of a water-soluble alkyd resin, a water-soluble, acrylic resin, a melamine formaldehyde resin, a urea formaldehyde resin, a polyvinyl acetatevinyl chloride copolymer Latex, a polystyrene-butadiene copolymer Latex and a vinyl chloride resin Latex as the binder resins, respectively, and in a manner similar to that described in this instant example. Then, these control electrophotographic copying materials were measured as regards their electrostatic characteristics and also were subjected to copying operation in a manner similar to that described in Example 1. The result was that these copying materials exhibited electrostatic characteristics which where were substantially the same with those indicated by the curve 3 in the drawing, but there was formed entirely no copied image at all.

Example 3 A binder solution having a composition identical to that of the solution (B) of Example 1 with the exception that the maleic acid monoester of polyvinyl acetate was substituted by itaconic acid monoester of polyvinyl acetate which was a 8% by mole monoesterified compound manufactured and sold under the trade name of A-75 (Z2) by the Nippon Synthetic Chemical Industry Co., Ltd., was prepared. This solution was placed, together with a photoconductive substance-dispersed solution having a composition identical to the solution (A) of Example 1, in a homogenizing mixer and they were dispersed and mixed for minutes. Thus, a solution for forming light-sensitive layers was prepared. Using the resulting solution in a manner similar to that described in Example 1, electrophotographic copying materials were obtained. The copying materials thus obtained were then measured as regards their electrostatic characteristics which were found to be of the same order with those which are indicated by the curve of the copying materials of the present invention in the drawing.

Example 4 By the use of a binder resin which was a mixture of 7 parts by weight of the binder resin used in Example 2, and 3 parts by weight of fumaric acid monoester of polyvinyl acetate which was a by mole monoesterified compound manufactured and sold under the trade name of E-50 (Z2) by the Nippon Synthetic Chemical Industry Co., Ltd., there was prepared a binder resin solution in such a way that the solution had a composition identical to that used in Example 2. By using this binder resin solution in a manner similar to that described in Example 2, electrophotographic copying materials were obtained. These copying materials were subjected to copying operation by the use of a copying apparatus similar to that used in Example 1, with the result that very clear copied images were obtained. These copying materials were measured of their electrostatic characteristics. The result, was that they exhibited electrostatic characteristics which were of the same order to those indicated by the curve exhibited by the product of the present invention in the drawing.

What is claimed is:

1. A method for manufacturing an electrophotographic copying material, comprising the steps of:

(a) dissolving a polybasic acid monoester of polyvinyl acetate in an aqueous solution containing hydrogen peroxide and at least one volatile basic substance selected from the group consisting of ammonia, triethylamine, trimethylamine, diethylamine, dimethylamine, ethylamine, methylamine,

methylethylamine, diisopropylamine and methyldiethylamine, said monoester being obtained by monoesterifying an acid selected from the group consisting of maleic acid, itaconic acid and fumaric acid with polyvinyl acetate;

(b) dispersing in the resultant solution a photoconductive substance selected from the group consisting of zinc oxide, titanium oxide, zinc sulfide, zinc selenide and cadmium sulfide;

(c) applying the resultant dispersion onto one surface of a support having an electroconductive propety; and then (d) drying the applied dispersion at a temperature ranging from to 200 C. for a period of time SLlfi'lClCIlt to evaporate substantially all of said volatile basic substance, thereby forming a photoconductive layer on said support.

2. A method according to claim 1, wherein said monoester is formed by esterifying polyvinyl acetate with from 2 mol percent to 30 mol percent of said acid.

3. In an electrophotographic copying material comprising an electroconductive support and a photoconductive layer on one surface of said support, said layer comprising a photoconductive substance dispersed in a hinder, the improvement in which said binder consists essentially of a polybasic acid monoester of polyvinyl acetate, and said photoconductive substance is selected from the group consisting of zinc oxide, titanium oxide, zinc sulfide, zinc selenide and cadmium sulfide, said monoester being obtained by monoesterifying an acid selected from the group consisting of maleic acid, itaconic acid and fumaric acid with polyvinyl aetate.

4. An eletrophotographic copying material according to claim 3, wherein said monoester has a three dimensional chemical structure formed by polymerization of said monoester.

References Cited UNITED STATES PATENTS 2,796,413 6/1957 Baer 260'.'8.4 3,471,625 10/1969 Adams et al. 961.5X 3,489,559 1/1970 Clark et al. 96l.5

FOREIGN PATENTS 1,434,759 2/1966 France 96l.5 1,030,725 5/1966 Great Britain 96l.5

GEORGE E. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner U.S. c1. X.R. 

